title: Velocity navigator for motion compensated thermometry.
abstract: OBJECTIVE
  Proton resonance frequency shift thermometry is sensitive to breathing motion that leads to incorrect phase differences. In this work, a novel velocity-sensitive navigator technique for triggering MR thermometry image acquisition is presented.
  
  
  METHODS
  A segmented echo planar imaging pulse sequence was modified for velocity-triggered temperature mapping. Trigger events were generated when the estimated velocity value was less than 0.2 cm/s during the slowdown phase in parallel to the velocity-encoding direction. To remove remaining high-frequency spikes from pulsation in real time, a Kalman filter was applied to the velocity navigator data. A phantom experiment with heating and an initial volunteer experiment without heating were performed to show the applicability of this technique. Additionally, a breath-hold experiment was conducted for comparison.
  
  
  RESULTS
  A temperature rise of ΔT = +37.3°C was seen in the phantom experiment, and a root mean square error (RMSE) outside the heated region of 2.3°C could be obtained for periodic motion. In the volunteer experiment, a RMSE of 2.7°C/2.9°C (triggered vs. breath hold) was measured.
  
  
  CONCLUSIONS
  A novel velocity navigator with Kalman filter postprocessing in real time significantly improves the temperature accuracy over non-triggered acquisitions and suggests being comparable to a breath-held acquisition. The proposed technique might be clinically applied for monitoring of thermal ablations in abdominal organs.
authors.affiliation: Medical Physics in Radiology (E020), German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120, Heidelberg, Germany. f.maier@dkfz.de
authors.forename: Florian
authors.initials: F
authors.lastname: Maier
authors.forename: Axel J
authors.initials: AJ
authors.lastname: Krafft
authors.forename: Joshua P
authors.initials: JP
authors.lastname: Yung
authors.forename: R Jason
authors.initials: RJ
authors.lastname: Stafford
authors.forename: Andrew
authors.initials: A
authors.lastname: Elliott
authors.forename: Rüdiger
authors.initials: R
authors.lastname: Dillmann
authors.forename: Wolfhard
authors.initials: W
authors.lastname: Semmler
authors.forename: Michael
authors.initials: M
authors.lastname: Bock
mesh_terms: D000465:Algorithms; D001831:Body Temperature; D017352:Echo-Planar Imaging / Q000379:methods; D004867:Equipment Design; D006358:Hot Temperature; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D015233:Models, Statistical; D009038:Motion; D019047:Phantoms, Imaging; D011522:Protons; D012119:Respiration; D013696:Temperature; D013997:Time Factors
chemical_list: D011522:Protons